Method of making sheathed electric heating units



May 4, 1954 s. A. OAKLEY 2,677,172

METHQD OF MAKING SHEATHED ELECTRIC HEATING UNITS Filed Oct. 16, 1948 INVENToR. Sterling A. Oakley Patented May 4, 1954 METHOD OF MAKING SHEATHED ELECTRIC HEATING UNITS Sterling A. Oakley,

New York Lincolnwood, Ill., assigner to General Electric Company,

a corporation of Application October 16, 1948, Serial No. 54,982

(Cl. .Z9-455.5)

9 Claims.

The present invention relates to methods of making sheathed electric heating units and more particularly to improved cold rolling steps that are employed in such methods. This application comprises a continuation in part of the cepending application of Sterling A. Oakley, Serial No. 483,699, filed April 19, 1943, now abandoned.

An electric heating unit of the sheathed type usually comprises an electric resistance conductor or element having a helical form, an enclosing tubular metal sheath, and a body of compressible heat-conducting and electricalinsulating material embedding the resistance conductor and holding it in a substantially central location within the sheath. Ordinarily, the opposits ends of the resistance conductor are welded cr otherwise secured to electrical terminals that are also embedded in the insulating material and respectively project from the opposite ends of the sheath. Finally, the opposite ends of the sheath may be sealed by glass or other suitable plugs respectively surrounding the projecting terminals. One electric heating unit of this type is disclosed in U. S. Patent No. 1,367,341, granted on February 1, 1921 to C. C. Abbott. Ordinarily, the resistance conductor is formed of a suitable nickel-chromium alloy, the sheath is formed of a suitable nickel-chromium-iron alloy that is sumciently plastic to permit cold working thereof, and the resistance material essentially comprises finely divided or granulated magnesia.

1n methods heretofore employed in making an electric heating unit of the type noted the parts mentioned have been assembled and then the sheath has been worked so as to reduce the cross sectional area thereof in order firmly to compact the insulating material in the resulting space between the resistance conductor and the wall of the sheath. This working of the sheath has been performed by swaging or hammering and has always been performed while the assembly occupied a horizontal position, since commercially available swaging machinery is designed to operate in this manner.

Applicant in his endeavor to improve the performance and to extend the useful life of heating units of this type observed that one of the principal factors causing ultimate failure of these units was lack of symmetry of the resistance conductors with respect to the sheaths and concluded that this defect was resulting from sagging of the resistance conductors from their assembled substantially central positions during the working of the sheaths in accordance with the prior methods that were employed in the manufacture oi the heating units.

K Accordingly, it is a general object of the present invention to provide an improved method of making an electric heating unit of the type noted that insures that in the finished heating unit the resistance conductor occupies a substantially central location within the sheath.

Another object of the invention is to provide an improved method of making an electric heating unit of the type noted, wherein the elements of the heating unit are assembled in a substantially upright position and then the sheath is cold worked in a substantially upright position, without permitting the sheath to assume an appreciable horizontal position following assembly of the elements thereinto and until after working thereof, so as to prevent any substantial sagging of the resistance conductor from its substantially central location within the sheath.

A further object of the invention is to provide an improved method of rolling an electric heating unit assembly of the type noted, wherein the assembly is subjected to successive gradual cold rolling passes while it occupies a substantially upright position and wherein the rolling passes are sufficiently gradual to prevent any substantial lateral displacement of the resistance conductor from its substantially central location within the sheath as the sheath is elongated and as the insulating material therein is compacted.

Further features of the invention pertain to the particular arrangement of the steps of the method, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which Figure 1 is a side elevational view, partly broken away, of a loaded sheathed heating unit assembly that is prepared for rolling in accordance with the method of the present invention; Fig; 2 is a reduced fragmentary perspective view of the heating unit assembly undergoing cold rolling in accordance with the present method, and illustrating schematically the first three rolling stages of a cold rolling machine that may be employed in carrying out the present method; Fig. 3 is an enlarged transverse sectional View, taken along the line 3--3 in Fig. 2,

rof the heating unit assembly preceding the first rolling stage of the rolling machine; Fig. 4 is an enlarged transverse sectional View, taken along the line 4-d in Fig. 2, of the heating unit assembly following the iirst rolling stage of the rolling machine, as Well as a fragmentary plan view of the rolls incorporated in the Erst rolling stage; Fig. 5 is an enlarged transverse sectional view, taken along the line lie-5 in Fig. 2, of the heating unit assembly following the second rolling stage of the rolling machine, as Well as a fragmentary plan View of the rolls incorporated in the second rolling stage; and Fig. 6 is an enlargedY transverse sectional view, taken along the line fil-t in Fig. 2, of the heating unit assembly following the third rolling stage of the rolling machine, as well as a fragmentary plan View of the rolls incorporated in the third rolling stage.

Referring now to the drawing, Fig. l there is illustrated a sheathed electric heating unit assembly Ill that has been prepared for the rolling operations. rlhis assembly it comprises an outer metallic tubular sheath Ii that may be formed of a suitable nichelchromiuzndron alloy and having a substantially circular crossmsection, and an electricalv reustance conductor or element l2' that may be formed of a suitable niclrelchromium alloy andy locatedY substantially eenm trally Within the sheath li and embedded in a body of heat-conducting and electrical-insuiating material I 3. The insulating material is in finely divided or granulated form and holds the resistance conductor' I2 its substantially tral location. Preferably the insulating it essentially comprises` and been charged into the sheath ll 'taznped in place. Preferably the opposite ends of the resiste conductor i2 are Welded or otherwise y secured to two electrical terminals It and that respectively project from the opposite ends of the sheath ihthe inner ends of the terminals lf:- and i5 being also embedded in the insulating rial lli. Finally, the opposite ends or the sheath l l are close-d by suitable Washers It and il respectively surrounding the 'terminals lli and l5 and retaining the insulating material it in place.

In producing the assembly is the loading chine disclosed in U. S. Patent No. granted on April 13, i943 to John L. Andrews, may be employed. More particularly, the inner ends of the terminals l are secured to the opposite ends of 'the resistance conductor il?, the lower Washer il is secured to the terminal these elements are threaded through the lower end of the sheath lli, the Washer il engag ing a locating bead. it formed adjacent 'to the lower of the sheath li. At this time the terminal lll held in a substantially central locan tion in the upper end of sheath il, the body of insulating material i3 charged into the sheath il While the sheath i l and the elen ments mentioned occupy' a substantially upri or vertical position. As the finely divided insu lating material I3 is charged into the sheath l i it embeds first the terminal iii, then the resistu ance conductor and ultimately the ter in l I-fl restraining or holding these elements their substantially central location within the sheath l I. .During charging of the insulating material I3 into the sheath Il', the sheath il may vibrated or jarred slightly in order to insure tamping or packing of the insulating material in the space between the terminals I /l and l5 and the resistance conductor I2 and the Wall of the sheath I I and between the convolutions of the resistance conductor I2 and into the core of the resistance conductor i2 denned by the convolurolls of the second rolling tions thereof. After the insulating material I3 has been charged into the sheath Il and tamped in place filling the spaces mentioned, the upper Washer le is secured in place Within the upper end of the sheath l I in order to prevent any possible escape of the insulating material i3 from the sheath iI. Ati this time the assembly it is removed from the loading machine to be subn seuuently finished in a cold rolling machine as described. below. In removing the assembly I0 from the loading machine and in storing the assembly lc' prior to inserting it in the rolling machine as described below, the assembly il) is retained in its substantially upright position in order positively to prevent sagging of the resistance conductor I2 prior to inserting the assezm bly Il! into the rolling machine.

At this time the assembly It is subjected to a plurality of successive gradual cold rolling passes, and in carrying out this operation the cold rolling machine disclosed in the previously mentioned Oakley application may be employed. As schematically illustrated in Fig. 2, this cold rolling machine comprises a plurality oi successive gradual oval rolling stages arranged in substantially vertical alignment and alternately angularly disapproximately ninety degrees. Specilically, this comprises a first rolling stage including the pair of individual forming rolls 2l, second rolling including the pair of indi- .al vforming rolls 22, a third rolling stage inindividual forming rollsv successive and similarV rolling stages, not shown. The first, second, third, etc., rolling i s are arranged in substantially vertical ornent, as previously noted, the first rollinl being lowerrnost in the rolling machine ociated with. guide structure, not shown, for the purpose of introducing the upper end of the assembly lil between the rolls 2, thereof. The roms ZI of the rolling the rolls '.53 ci the third rolling and the other rolls of the other odd rolling, stages are arranged a irst substantially vetrically disposed plane; while the stage, and the other of the other even rolling stages are arranged in a second substantially vertically disposed plane; wherein the irst and second planes .menu tioned are angularly displaced approriin'lately ninety' degrees with respect to each other. The rolls 2! in the iirst rolling stage have complen mentary substantially semi-elliptical forming grooves 2id arranged therein and coopera-t1 lg to denne a pass having a generally elliptical cross-secticn; the rolls E2 in the second rolling stage have complementary substant ly semi elliptical forming grooves 22o ed therein and cooperating to deine a second pass having a generally elliptical cross-sectiong the rolls 23 in the thirr. rolling stage have coin lementary substantiall'y semi-elliptical forming grooves tta, are ranged therein and" cooperat' ig to denne a third pass having a generally ellipticaly cross-sectionthe first', second; third, etc., passes are 1' ductive of ellipses of gradually reduc-'ed' cross sectional areas and angularly rotated approxi-y mately ninety degrees w-ith respec' to each other. The nal rol-lingV stage, or stages, not shown, incorporating in` the roll-ing machine may beV pro14V ductive of a substantially cylindrical pass, although any other` desired conliguration may be produced by providing appropriate complementary forming grooves in the cooperating rolls thereof.

In this rolling. machine each rolling stage is provided with an individual adjustable speed drive, not shown, whereby the second, third, etc., rolling stages may be operated at successively slightly increased speeds with respect to the speed of the nrst rolling stage in order to accommodate elongation of the assembly I as the cross sectional area thereof is successively reduced without buckling of the assembly I0 between the successive rolling stages. Also the individual rolls of each rolling stage are rotated in opposite din recticns and in such directions as to propel or feed the assembly i0 upwardly through the rolling machine after the upper end of the assembly it has been inserted between the rolls 2l of the first rolling stage.

Accordingly, in the operation of this rolling machine it will be understood that after the upper end of the assembly I0 is inserted by the guide structure, not shown, between the complenienn tary forming grooves 2id formed in the rolls il of the first rolling stage that the assembly it is moved or fed upwardly through the rolling machine and ultimately discharged from the upper end thereof. The passes incorporated in the rolling machine are suciently gradual to prevent appreciable flowing of the insulating material i3 ahead of the individual rolling stages and the consequent longitudinal distortion of the convolutions of the resistance conductor I2. Moreover, the passes incorporated in the rolling machine are sumciently gradual in order successively to reduce the cross sectional area of the assembly without appreciable lateral distortion ci the conyolutions of the resistance conductor i2 or lateral displacement of the resistance conductor I2 from its substantially central location within the sheath l l. As clearly indicated in Figs. il, 5, and 6, the orientation of the major axes of the successive elliptical cross-sections produced in the assembly I0 at the respective rst, second and third rollmg stages are alternately rotated by approximately ninety degrees, which arrangement is very advantageous in that the two rolling stages respectively disposed below and above a given rolling stage constitute vises for holding the respective upper and lower ends of the section of the assembly it that is being rolled in the given rolling stage. Moreover, this arrangement prevents the production of fins or ridges on the sheath Il at the junctions between the cooperating forming grooves provided in the different pairs of rolls inthe various rolling stages. Finally, in order positively to insure that the outer surface of the sheath EI is not damaged incident to performing the rolling operations described the outer surface of the sheath l l may be given a preliminary thin coating of oil, graphite or other suitable drawing compound.

During the operation of the machine the sheath i l is elongated and the cross sectional area thereof is reduced so as to compact the insulating material i3 into a dense mass and firmly into engagement wtih the resistance conductor l?. and the terminals Ill and l5 and firmly in the resulting space between these elements and the wall of the sheath ll and between the convolutions of the resistance conductor I2. After the assembly it has been discharged from the cold rolling machine it is annealedand may then be stored in a horizontal or any desired position until it is ultimately finished for use. In the nishing operations the opposite ends of the sheath I i are stripped back in order to remove the Washers lo and il. rlhereafter the opposite ends of the compacted dense mass of insulating material I3 may be cored in order to provide cavities respectively adjacent to the opposite ends of the sheath II and respectively surrounding the terminals I4 and Then these cavities may be filled with glass plugs in order to seal the opposite ends of the sheath l l and to support the terminals I4 and I5 in their substantially central locations projecting from the opposite ends of the sheath i I. Finally, the sheath li of the finished assembly I0 may be nattened and otherwise formed to provide the required conguration of the completed heating unit.

For example, in rolling one standard heating unit assembly l0 the sheath Il is formed of Inconel, an alloy comprising approximately 78% nickel, 13% chromium and 9% iron; and the cold rolling machine comprises eight rolling and forming stages. The sheath II is initially substantially cylindrical having an outside diameter of approximately 0.373". The first rolling stage is substantially elliptical, whereby the first oval in the sheath II has major and minor outside dimensions of approximately 0.379" and 0.355", respectively. The second rolling stage is substantially elliptical, whereby the second oval in the sheath Il has major and minor outside dimensions of approximately 0.369" and 0.346, respectively. The third rolling stage is substantially elliptical, whereby the third oval in the sheath ii has major and minor outside dimensions of approximately 0.359" and 0.337", respectively. lllhe fourth rolling stage is substantially elliptical, whereby the fourth oval in the sheath I I has major and minor outside dimensions of approximately 0349 and 0.327", respectively. The fth rolling stage is substantially elliptical, whereby the fifth oval in the sheath II has major and minor outside dimensions of approximately 0.339 and 0.318", respectively. The sixth rolling stage is substantially elliptical and identical to the fifth rolling stage and is provided primarily to take up any wear in the preceding rolling stages, whereby the sixth oval in the sheath II again has major and minor outside dimensions of approximately 0.339 and 0.318, respectively. The seventh and eighth rolling stages are substantially circular and identical and insure that the finished sheath II is smooth and round and of an outside diameter of approximately 0.333. Thus the outside diameter of the sheath Il is reduced from 0.373" to 0.333 in the several cold rolling passes effecting a corresponding reduction in the cross sectional area thereof, this reduction in the initial cross sectional area of the sheath Il being in excess of 21% in the present example. By employing the alloy mentioned in the sheath Il and the gradual rolling passes described, not only are the previously mentioned characteristics attained in the finished heating unit, but it is pointed out that annealing between the successive cold rolling passes is not required in working the metal sheath II.

The utilization of the cold rolling machine disclosed in the previously mentioned Oakley application for the purpose of carrying out the rolling steps described is very advantageous in view of the fact that the successive gradual oval passes are accomplished by a single feeding of the assembly I0 in an upright position therethrough; however, it will be understood that it is not imperative that this rolling machine be employed, as the assembly I0 may be passed successively in an upright position through a cold rolling machine provided with a single rolling stage, with acvmve 9 heating unit including a tubular metal sheath, an electrical resistance conductor substantially centrally located within said sheath, and a body of iinely divided oompressible heat-conducting and electrical-insulating material embedding said resistance conductor and filling the space between said resistance conductor and the wall of said sheath; the steps comprising producing an assembly or said elements named, and then subjecting said sheath to a plurality oi successive gradual cold rolling passes while said assembly is arranged in a substantially upright position so as to reduce successively and gradually the initiai cross sectional areaJ of said sheath by at least 16% in order to compact said material into a dense mass in the resulting space between said resistance conductor and the wall of said sheath without appreciable lateral displacement oi said resistance conductor from its substantially central location within said sheath.

5. In the method of making a sheathed electric heating unit including a tubular metal sheath, a helical electrical resistance conductor substantially centrally located within said sheath, and a body of finely divided compressible heat-conducting and electrical-insulating material embedding said resistance conductor and nlling the space between said resistance conductor and the wall oi" said sheath and the space between the convolutions of said resistance conductor; the steps comprising producing an assembly of said elements named, and then subjecting said sheath to a plurality of successive gradual cold rolling passes while said assembly is arranged in a substantially upright position so as to reduce successively and gradually the initial cross sectional area of said sheath by at least 10% in order to compact said material into a dense mass in the resulting space between said resistance conductor and the wall of said sheath and between the convolutions of said resistance conductor without appreciable lateral displacement of said resistance conductor from its substantially central location within said sheath and without appreciable distortion of the convolutions of said resistance conductor.

6, In the method of making a sheathed electric heating unit including a tubular metal sheath, an electrical resistance conductor substantially centrally located within said sheath, and a body of iinely divided compressible heat-conducting and electrical-insulating material embedding said resistance conductor and filling the space between said resistance conductor and the wall of said sheath; the steps comprising producing an asil sembly of said. elements nam-ed, and then cold rolling said sheath in successive gradual oval passes while said assembly is arranged in a substantially upright position so as to reduce successively and gradually the initial cross sectional area of said sheath by at least 10%, and changing the relative orientation of said sheath and said oval passes between successive ones of said oval passes in order alternately to rotate the major axis of the oval cross section of said sheath through a substantial angle between successive ones of said oval passes, thereby to compact said material into a dense mass in the resulting space between said resistance conductor and the wall of said sheath.

1. in the method of making a sheathed electric heating unit including a tubular metal sheath, a helical electrical resistance conductor substantially centrally located within said sheath, and a body of iinely divided compressible heat-con- "ducting and electrical-insulating material embedding said resistance conductor and filling the space between said resistance conductor and the wall of said sheath and the space between the convolutions of said resistance conductor; the steps comprising producing an assembly of said elements named, and then cold rolling said sheath in successive gradual oval passes while said assembly is arranged in a substantially upright position so as to reduce successively and gradually the initial cross sectional area of said sheath by at least i0 and changing the relative orientation of said sheath and said oval passes between successive ones o said oval passes in order alternately to rotate the major axis of the oval cross section oi said sheath through an angle of approximately .90 between successive ones of said oval passes, thereby to compact said material into a dense mass in the resulting space between said resistance conductor and the wall of said sheath and between the convolutions of said resistance conductor.

8. In the method of making a sheathed electric heating unit including a tubular metal sheath, a helical electrical resistance conductor substantially centrally located within said sheath, and a body or nnely divided compressible heat-conducting and electrical-insulating material embedding said resistance conductor and filling the space between said resistance conductor and the wall oi said sheath and the space between the convolutions or" said resistance conductor; the steps comprising producing an assembly of said elements named, and then moving said assembly upwardly while arranged in a substantially upright position and simultaneously subjecting said sheath to a plurality of successive gradual angularly rotated cold rolling passes so as to reduce successively and gradually the initial cross sectional area of said sheath by at least 10% in order to compact said material into a dense mass in the resulting space between said resistance conductor and the wall of said sheath and between the convolutions of said resistance conductor without appreciable lateral displacement of said resistance conductor from its substantially central location within said sheath and without appreciable flowing of said material ahead of said rolling passes and the consequent longitudinal distortion oi the convolutions of said resistance conductor.

9, The method of making a sheathed electric heating unit including a tubular metal sheath, a helical electrical resistance conductor substantially centrally located within said sheath, and a body of compressible heat-conducting-electricalinsulating material embedding said resistance conductor and holding it in its substantially central location within said sheath, comprising the steps of producing an assembly of said resistance conductor within said sheath, placing said assembly in a substantially vertical position in a vertical material charging and tamping machine, operating said machine to charge said material in nnely divided form into said sheath about said resistance conductor and to tamp said material in place between said sheath and said resistance conductor in order substantially centrally to locate said resista-nce conductor within said sheath, transferring said assembly in a substantially vertical position from said machine to a vertical plural stage cold rolling mill, and then operating said mill to move said assembly in the vertical direction and simultaneously to subject said sheath to a plurality of successive gradual successively References Cited in the le of this patent ectlonal area 0f UNITED STAT-Es PA'ITENTS dense mass in the resulting Number 'Name Date said resistance conduct@ and the 5 132,923 H2111 Oct. 3, 1876 @preamble lateral 184,719 Loring Nov. 2e, 1876 L o1 said resistance conductor from 15885558 T hol'nton June 15, 1926 on Within Sad 1,662,680 Lmdgren Mar. 13, 1928 `distortion of the 1,858,990 Foren May 17, 1932 10 2,063,642 Vanden Berg Dec. 8, 1936 2,483,839 Oakley Oct. 4, 11949 

